In process automation technology, field devices are often employed, which serve to register and/or influence process variables. Serving for registering process variables are sensors, such as, for example, fill-level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH-redox potential measuring devices, electrical conductivity measuring devices, etc., which register the respective process variables, fill-level, flow, pressure, temperature, pH-value and conductivity. Serving for influencing process variables are actuators, for example valves or pumps, via which the flow of a fluid in a section of pipeline or the fill-level in a container can be changed. In principle, all devices which are employed near the process and which deliver or work with process-relevant information are referred to as field devices. In addition to the aforementioned sensors and actuators, generally, units that are directly connected to a fieldbus and which serve to communicate with the superordinated units (e.g. remote I/Os, gateways, linking devices and wireless adapters) are also referred to as field devices. A large number of these devices are produced and sold by the Endress+Hauser Group.
In modern industrial facilities, field devices are, as a rule, connected with superordinated units via fieldbus systems (e.g. Profibus®, Foundation Fieldbus®, HART®, etc.). Normally, the superordinated units involve control systems or control units, for example a PLC (programmable logic controller). The superordinated units are used, for example, for process control, process visualizing, process monitoring as well as in the start-up of the field devices. The measurement values registered by the field devices—especially from the sensors—are transmitted via the connected bus system to a superordinated unit, or, as the case may be, to several superordinated units. Additionally, a transfer of data from the superordinated unit to the field devices via the bus system is necessary; this is used especially in the configuring and parametering of field devices or for diagnostic purposes. Generally speaking, the field device is serviced from the superordinated unit via the bus system.
In addition to a hardwired data transmission between the field devices and the superordinated unit, the possibility of a wireless data transmission also exists. In particular in the case of the bus systems Profibus®, Foundation Fieldbus® and HART®, a wireless data transmission via radio is specified. Additionally, radio networks for sensors are more precisely specified in the standard IEEE 802.15.4. For the realization of a wireless transmission of data, field devices are designed for example as radio-field devices. As a rule, these exhibit a radio unit and an electrical current source as integral components. In such a case, the radio unit and the electrical current source can be provided in the field device itself, or in a radio module which is permanently connected to the field device. Through the electrical current source, an autarkic energy supply for the field device is made possible.
Furthermore, there exists the possibility to equip field devices without radio units—i.e. the current installed base in the field—to become a radio-field device through the attachment of a wireless adapter which features a radio unit. A corresponding wireless adapter is described, for example, in the international publication WO 2005/103851 A1. The wireless adapter is, as a rule, connected to a fieldbus communication interface of the field device in a detachable manner. Via the fieldbus communication interface, the field device can transmit data over the bus system to the wireless adapter, which then transmits this via radio to the target location. Conversely, the wireless adapter can receive data via radio and forward it over the fieldbus communication interface to the field device. The supplying of the field device with electrical power then occurs as a rule via an energy supply unit associated with the wireless adapter.
@In the case of autarkic radio field devices and wireless adapters, the communication (for example with a superordinated unit) is, as a rule, conducted via a wireless interface of the radio field device or the wireless adapter. Additionally, such radio field devices or wireless adapters exhibit as a rule a hardwired communication interface. The HART Standard, for example, provides that the radio field device must, in addition to a wireless interface, also feature a hardwired communication interface. Via such a hardwired communication interface, an on-site configuration of the radio field device or wireless adapter is, for example, possible via a service or operating unit (for example a handheld communicator) which is connected to the hardwired communication interface. Furthermore, the hardwired communication interface can be embodied as a fieldbus communication interface, so that the communication is conducted through it according to a bus system, e.g. according to one of the standardized bus systems such as Profibus, Foundation Fieldbus or HART. Through such a fieldbus communicating interface, the radio field device or wireless adapter can also be connected to a corresponding hardwired fieldbus.
The energy supply unit or electrical current source of a wireless adapter or a radio field device is normally a battery. The charge status of batteries is, according to the state of the art, determined via a measurement of consumption, which is performed by means of a coulomb counter. Performing a so-called end of life (EOL) detection is also known. The corresponding components are available on the market.
Disadvantageous for determining the remaining service life of the battery from the measurement of consumption is the relatively high inaccuracy. This is especially the case if the charge status of the battery is not precisely known at the beginning of its use—a problem which arises, for example, in the case of a battery which has previously been used, or due to the differing charge statuses which also occur in the case of unused batteries.
The problem encountered in EOL detection can be seen in that in the case of batteries with a flat characteristic curve (U/t), a reliable prediction of the remaining service life is not possible. Additionally, further demand is made on the battery by the EOL detection.